AGC for gr-satellites

In a previous post I discussed my BER simulations with the LilacSat-1 receiver in gr-satellites. I found out that the "Feed Forward AGC" block was not performing well and causing a considerable loss in performance. David Rowe remarked that an AGC should not be necessary in a PSK modem, since PSK is not sensitive to amplitude. While this is true, several of the GNU Radio blocks that I'm using in my BPSK receiver are indeed sensitive to amplitude, so an AGC must be used with them. Here I look at these blocks and I explain the new AGC that I'm now using in gr-satellites.

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ÑuSat finally decoded

More than a year ago, I spoke about my efforts to decode ÑuSat-1 and -2. I got as far as reverse-engineering the syncword and packet length, and I conjectured that the last 4 bytes of the packet were a CRC, but without the scrambler algorithm I couldn't do much. Recently I've been exchanging some emails with Gerardo Richarte from Satellogic, which is the company behind the ÑuSat satellites. He has been able to provide me the details of the protocol that I wasn't able to reverse engineer. The result of this exchange is that a complete decoder for ÑuSat-1 and -2 is now included in gr-satellites, together with an example recording. The beacon format is still unknown, but there is some ASCII data in the beacon. Here I summarise the technical details of the protocol used by ÑuSat. Thanks to Gerardo for his help and to Mike DK3WN for insisting into getting this job eventually done.

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Acquisition and wipeoff for JT9A

Lately, I have been playing around with the concept of doing acquisition and wipeoff of JT9A signals, using a locally generated replica when the transmitted message is known. These are concepts and terminologies that come from GNSS signal processing, but they can applied to many other cases.

In GNSS, most of the systems transmit a known spreading sequence using BPSK. When the signal arrives to the receiver, the frequency offset (given by Doppler and clock error) and delay are unknown. The receiver runs a search correlating against a locally generated replica signal which uses the same spreading sequence. The correlation will peak for the correct values of frequency offset and delay. The receiver then mixes the incoming signal with the replica to remove the DSSS modulation, so that only the data bits that carry the navigation message remain. This process can be understood as a matched filter that removes a lot of noise bandwidth. The procedure is called code wipeoff.

The same ideas can be applied to almost any kind of signal. A JT9A signal is a 9-FSK signal, so when trying to do an FFT to visually detect the signal in a spectrum display, the energy of the signal spreads over several bins and we lose SNR. We can generate a replica JT9A signal carrying the same message and at the same temporal delay than the signal we want to detect. Then we mix the signal with the complex conjugate of the replica. The result is a CW tone at the difference of frequencies of both signals, which we call wiped signal. This is much easier to detect in an FFT, because all the energy is concentrated in a single bin. Here I look at the procedure in detail and show an application with real world signals. Recordings and a Python script are included.

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D-SAT image downlink

In a previous post, I spoke about the cubesat D-SAT. The thing that first caught my attention about this satellite is its image downlink and the quality of some of the images that Mike DK3WN has managed to receive. Yesterday, Mike sent me an IQ recording of D-SAT downlinking a couple of images. After using the Groundstation software by the D-SAT team to verify that the images in the recording can be decoded, I have reverse engineered the protocol used to transmit images and added an image decoder to the D-SAT decoder in gr-satellites.

The image decoder can be tested with the dsat-image.wav recording in satellite-recordings. This WAV file contains the image below, which shows the Southwestern part of Spain and Portugal. The image was taken by D-SAT on 2017-08-17 10:09:54 UTC and received by Mike during the 19:10 UTC pass that evening.

Image of Spain and Portugal taken by D-SAT

According to the TLEs, at the time this image was taken, D-SAT was just above Rincón de la Victoria, in Málaga, passing on a North to South orbit. This means that D-SAT's camera was pointing more or less in a direction normal to the orbit.

This image is a 352x288 pixels JPEG image with a size of 13057 bytes. It took 43 seconds to transfer using D-SAT's 4k8 AF GMSK downlink (yes, the overhead is around 100%, more on that later). In the rest of this post, I detail the protocol used to transmit the images.

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IPv6 for Amateur radio

Back in September 26 2016, I posted an email in Spanish to the Hamnet.es mailing list detailing my proposal for an IPv6 Amateur radio network, and trying to engage people into some preliminary tests. In October 1 2016, I posted a summary (in English) of my message to the 44net mailing list. There was some discussion afterwards in the list, but no real actions were taken. Since then, no much interest in IPv6 for Amateur radio seems to have sprung. Still, I think that the time for IPv6 will come. I have collected my IPv6 for Amateur radio proposal in a page here for future reference. At least I hope that this pops up on Google searches for IPv6 and Amateur radio, since there is not much material about this on the Internet, and most of what one can find is quite dated.

D-SAT support added to gr-satellites

D-SAT is an Italian cubesat that will demonstrate a new deorbit hardware. Apparently this system uses dedicated propulsion to make the satellite re-enter from a 500km orbit in 30 minutes. It also carries three more experiments and it was launched in June 23 together with several other small satellites. According to the information from the team, it transmits 4k8 telemetry in the 70cm band. It is not stated explicitly, but we read attentively, we see that it uses a NanoCom U482C transceiver from GOMspace.

Recently, I have seen Mike DK3WN decode very nice images from D-SAT and I have investigated a bit to see what software he is using.

The satellite team provides some decoding software through their forum, which requires registration. Version 2 of their software can be downloaded directly here using the password dsatmission. Its software is based on GNU Radio and it uses a few components from gr-satellites, namely the U482C decoder and some KISS and CSP blocks. These have been incorporated into their decoder from before gr-satellites was restructured. They include a note thanking me in the README, but I didn't ever hear from them that they were using gr-satellites. It would have been nice if they had contacted me, since this opens up many possibilities for collaboration.

Apart from that, they include a groundstation software which performs telemetry decoding and so on. Unfortunately, the groundstation software is closed-source, distributed only as an x86_64 Linux executable. This is not good for Amateur Radio. We should strive for open source software and open specifications for everything that transmits in our bands. The groundstation software is also distributed in a quite ugly manner as the remains of an Eclipse project (source code stripped, of course). However, it is interesting because it seems that this software is the same they use in their groundstation, and it supports sending commands to the satellite. Naturally, the command transmission is not implemented in the software they distribute, but it is still very interesting to have a peek and see what kinds of commands the satellite supports.

I have added a D-SAT decoder to gr-satellites. The decoder supports sending frames to their groundstation software. Here I describe how to set everything up.

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BER simulation in GNU Radio

David Rowe always insists that you should simulate the bit error rate for any modem you build. I've been intending to do some simulations of the decoders in gr-satellites since a while ago, and I've finally had some time to do so. I have simulated the performance of the LilacSat-1 decoder, both for uncoded BPSK and for the Viterbi decoder. This is just the beginning of the story, as the code can be adapted to simulate other modems. Here I describe some generalities about BER simulation in GNU Radio, the simulations I have done for LilacSat-1, and the results.

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Degradation bug in GNU Radio "Decode CCSDS 27"

I am doing some BER simulations with GNU Radio (stay tuned for the next post), and during my experiments I have stumbled upon a bug in the "Decode CCSDS 27" block. This block is a Viterbi decoder for the CCSDS convolutional code with r=1/2, k=7 (note that the convention used by this block is first POLYA then POLYB so it doesn't match the NASA-DSN convention nor the CCSDS/NASA-GSFC conventions, as I have mentioned in another post).

The bug consists in the block entering a "degraded" state after it has processed many symbols (on the order of several millions). In this degraded state, it doesn't decode properly, producing lots of bit errors even if no input symbols are in error. Fortunately, there is another block in GNU Radio which can decode the CCSDS convolutional code, the "CC Decoder" included in FECAPI. This block doesn't seem to suffer this issue. Here I describe how to replicate the bug, how to replace "Decode CCSDS 27" by "CC Decoder" and some other miscellaneous things related to this bug.

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A first look at DSLWP SSDV downlink

The Chang'e 4 is a Chinese lunar mission that will land a rover on the far side of the Moon by the end of 2018. To support this mission, the Chang'e 4 relay satellite will be launched six months before and put into a halo orbit around the Earth-Moon Lagrange L2 point. The relay will provide four 256Kbps links with the rover and lander on X-band and a 2Mbps link with Earth on S-band using a 4.2m dish. Two CE-4 microsatellites will be launched together with the relay satellite. They will be put in a 200km x 9000km lunar elliptical orbit. The main mission of the CE-4 microsatellites is to perform HF interferometry of celestial bodies, using the Moon as a shield from the radiation of the Sun and Earth. The satellites also carry an Amateur radio system called DSLWP, which will provide telecommand, telemetry and image downlink.

A team at Harbin Institute of Technology is currently designing the Amateur radio payload. As it is the case with previous HIT satellites such as BY70-1 and LilacSat-1, the payload will have a camera which can be telecommanded by radio Amateurs, which can use it to take and download pictures. Yesterday, Wei BG2BHC has released some work in progress of the image downlink. Many important parts of the downlink will still change, but releasing the work in progress at this early stage is a very good idea. Probably it is not too late in the development process so that the Amateur community can contribute with ideas and improvements.

The release consists of an IQ recording of the signal containing a full image and a decoder in gr-lilacsat. The IQ recording is at 2ksamp/s, since the signal is FSK at 250baud. Note that the recording is almost 32 minutes long. It takes a while to transmit an image at such a low rate. However, a low baudrate and a good amount of FEC are needed for an effective downlink from the Moon, given the huge path loss of around 197dB in the 70cm band.

The good news about this work in progress is that SSDV is now used to transmit the image. SSDV is a packetised protocol based on JPEG, but which is tolerant to packet loss. In contrast, BY70-1 and LilacSat-1 send JPEG images in 64byte chunks, and a single lost chunk can destroy the image completely. SSDV was originally developed to transmit images from Amateur high altitude ballons, so it is a good idea to use it also for DSLWP.

The bad news is that the way that SSDV has been included into the downlink protocol is not very optimal. In the rest of this post I do an in-depth look at the protocol, point out the main problems and suggest some solutions. Hopefully the protocol can still be modified and improved.

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LilacSat-1 image downlink

Yesterday, Wei BG2BHC posted on Twitter an IQ recording of LilacSat-1 sending an image. LilacSat-1 has an onboard camera and it can send images using the same format as BY70-1. However, one has to keep in mind that in LilacSat-1 the Codec2 frames and the KISS stream with telemetry and image packets are multiplexed as described here, whereas BY70-1 only transmitted the KISS stream with telemetry and image packets. As in the case of BY70-1, the camera is potentially open to telecommand by all Amateurs, although it seems that system is not enabled yet.

The signal in Wei's recording is very strong and stable, about 20dB SNR in its natural bandwidth of 13kHz. Therefore, it is no surprise that the image can be decoded without errors.

When BY70-1 was in orbit, it was quite difficult for an Amateur station to get a perfect decode of the image, since a single fade in the signal would completely corrupt the JPEG file. LilacSat-1 doesn't seem particularly stronger than BY70-1, so the same degree of difficulty can be expected. Of course, a well equipped groundstation such as the one in Harbin Institute of Technollogy will have no problems to get a good decode, as shown by this IQ recording. Amateurs with more modest stations should resort to a collaborative effort to try to combine the different packets that form the image, as received by several stations. Currently this procedure can only be partially automated by software, because the CRC algorithm used in LilacSat-1 is not publicly known, so it is not possible to check the packets for bit errors.

LilacSat-1 image 143

The image transmitted by LilacSat-1 can be seen above. Its size is 13861 bytes and it took 217 camera packets and 1 minute and 26 seconds to transmit. This is pretty good, as it means that several images can be taken and transmitted during a pass.

Recall that the downlink of LilacSat-1 transmits at 4800bps, but 1400bps are taken for Codec2, leaving 3400bps for the KISS stream containing image packets (and telemetry packets). Each camera packet contains a 64 byte JPEG chunk, but taking into account headers it is 87 bytes long. We also need to take into account the overhead of the KISS stream. Assuming that no bytes have to be escaped, we just need to include 2 extra bytes for the frame delimiters, so a camera packet takes 89 bytes from the KISS stream and so it takes 197ms to transmit. This means that the image above could have been sent in only 43 seconds. All the extra time is probably due to the fact that the image was sent interleaved with many telemetry packets, although it would be interesting to examine if the KISS stream was in fact completely busy all the time during the image download.

The complete telemetry log decoded from this recording is in this gist. I have also taken the GPS data from the telemetry and plotted it in the map below. The position of the Harbin Institute of Technology, where the recording was made, is also shown.

A 48kHz WAV file extracted from the recording has been included in satellite-recordings. It can be fed directly to the gr-satellites LilacSat-1 decoder.